"The Klingon died."
tlh is considered a single Klingon consonant. It, also, will always have at least one adjacent vowel. Though in English we write tlh with an "l" in the middle, the line you have quoted is referring to the separate Klingon consonant l, and is meant to exclude the "l" in the middle of the Klingon consonant tlh (which appears in many Klingon words). I hope that is more clear.